At underground stations it is today a problem that different objects or persons may be present on the track and cause accidents when an underground train arrives at the station. The underground station is frequently short in relation to the speed of the train. Furthermore, the driver of the train normally has no possibility to see what happens at the platform, i.e. beside the track proper. Consequently, the driver has no real possibility to brake the train in good time if any object moves from the platform out on the track when the train arrives at the station. At railway stations and railway crossings, there are similar problems with persons or objects, for instance vehicles, which can be present on the track when a train arrives. Also in other areas such as airports, ports, parking places and the like there are similar problems with undesired objects or persons, for instance intruders.
Many of the areas mentioned above are too difficult to monitor due to the fact that the area lacks a physical limitation. Automatic system frequently can not delimit the monitoring to a selectable, determined area, but objects and persons moving outside the area to be monitored will disturb the signal obtained
DE-10104157 discloses a monitoring arrangement including a relatively large number of vertical laser devices for monitoring a respective door of an underground train, and a relatively large number of horizontal laser devices for monitoring the track. The horizontal laser devices are arranged after each other beneath the platform along the track at an underground station. Furthermore, each of the horizontal laser devices is intended to sense a respective limited field or area of the track for identifying possible objects on the track. The laser devices are connected to a computer for evaluating the sensed fields. The arrangement disclosed has the disadvantage that it is complex and expensive to install at least partly due to the large number of laser devices to be mounted and connected.
JP 2000-127972 discloses the possibility of providing a photographic member or a laser for monitoring a platform and a track area. In this case, physical markings have been arranged at the platform for facilitating the reading of the reflected light.
U.S. Pat. No. 5,787,369 discloses a further system for monitoring a track area at a railway or a railway crossing. The system includes a laser arranged to scan an area.
WO93/154160 discloses a device for monitoring railway crossings, i.e. a relatively small area.
WO01/53851 discloses a radar monitoring device for detecting objects at a track area. The known device may be applied to an underground system
U.S. Pat. No. 5,500,525 discloses a system for watching a predetermined, limited area by means of a laser beam. The known system includes an algorithm based on a comparison of an actual reflected image and a stored reference image. The system also includes an identification of an object that has been detected within the area.
U.S. Pat. No. 4,952,911 refers to an intruder watching system for watching an area. The system includes a laser device including an IR-laser diode operating at an effect of 30 mW and at a wavelength spectrum with a peak at 830 nm. The system also includes means for determining the distance to the point from which the laser beam is reflected by means on triangulation. U.S. Pat. No. 4,949,074 discloses a similar intruder watching system.
US2003/0036881 discloses a method and a system for detecting an object. The device uses a light source in the form of a laser operating within a wavelength range between 700-1500 mm. The device disclosed is intended to detect a thin area straight ahead and includes no means for scanning the beam over a larger area. Nothing is said about the possibility of using the device for monitoring a rail area or for instance an airport.